Scale deposits frequently occur in the production of water, oil and gas from subterranean formations and can result in plugged well bores, plugged well casing perforations, plugged tubing strings, stuck downhole safety valves as well as other valves, stuck downhole pumps and other downhole and surface equipment and lines, scaled formations and fractures in the vicinity of the well. Scale formation can occur as a result of mixing incompatible waters in the well which produce precipitates, or as a result of temperature and pressure changes in the produced waters during production. Generally, incompatible waters occur in water-flooding, as injected sea water mixes with formation water in the borehole during water breakthrough. The more common concern is scale deposited due to changes in supersaturation or solubility of minerals in the formation or produced waters caused by pressure and temperature changes, or changes in other physical and chemical parameters, such as gas compositions, ratio of gas/oil/water. Scale may also be formed from corrosion of metal equipment used in the subterranean oil and gas production. Scale formation is also a problem in aqueous systems used in cooling towers, boilers and the like. Precipitation frequently encountered as scale includes calcium carbonate, calcium sulfate, barium sulfate, magnesium carbonate, magnesium sulfate, and strontium sulfate.
Scale formation can be reduced by the introduction of inhibitors into the formation. Various inhibitors are known, including carboxylated polymers, organophosphates, and polyphosphonates. Typically, carboxylated polymers are polymers and copolymers of acrylic or methacrylic acids, commonly referred to as polyacrylic acids. Organophosphorous-containing inhibitors include alkyl ethoxylated phosphates; ethylenediaminetetramethylene phosphonic acid; aminotrimethylene phosphonic acid; hexamethylenediaminetetramethylene phosphonic acid; diethylenetriaminepentamethylene phosphonic acid; hydroxyethylidenediphosphonic acid and polyvinyl phosphonic acid. The injection of scale inhibitors without pre or post cross-linking to protect an oil or gas well from mineral scale formation is widely practiced. However, such treatments often result in poor retention in the subterranean formation, quick depletion and frequent re-treatments.
Often scale inhibitors are injected into a wellbore to prevent scale from forming along the vertical interval of a wellbore that fluidly communicates with the formation via perforations. Frequently, these intervals contain permeabilities that vary from each other. Desired scale inhibitors are usually injected into the wellbore at rates and pressures sufficient to pump the inhibitor into the intervals at pressures below fracturing pressures. When intervals or zones have permeability variations, an uneven distribution of the inhibitor can result.
Therefore, what is needed is a method to effectively place a scale inhibitor evenly into intervals of a formation where said intervals contain varying permeabilities.